The application generally relates to control systems for chilled liquid systems. The application relates more specifically to a control process to pull the leaving chilled liquid temperature of a chilled liquid system down to a predetermined set point.
In a chilled liquid system, a chilled liquid, for example, water, is circulated through a building to remove heat from the building. The liquid in the chilled liquid system is cooled in an evaporator (or chiller) using a heat exchanger, wherein the liquid is cooled by a refrigerant that accepts heat from the liquid. Chilled liquid systems are often operated at temperatures (operating setpoints) within a few degrees of the freezing point of the liquid. In conventional chilled liquid systems, a control system is often programmed to shut down the chilled liquid system as soon as the evaporator liquid temperature decreases to a certain temperature (a cutout point) near or below the freezing point of the liquid to avoid freezing the tubes and damaging the evaporator. For example, a chilled liquid system using water may have an operating setpoint of 35 degrees F. and a cutout point of 34 degrees F.
A difference of only one degree between the operating setpoint and the cutout point in the chilled liquid system generally does not cause problems during normal or steady-state operation of the chilled liquid system. However, when the chilled liquid system is required during a pull-down operation to reduce the liquid temperature from an ambient temperature to the operating setpoint, a one degree difference between the operating setpoint and the cutout point can be problematic. The chilled liquid system may be shut down by the control system during a pull-down operation, if the liquid temperature in the evaporator overshoots the operating setpoint and reaches the cutout point. To avoid this problem, conventional control systems are programmed to minimize overshoot during pull-down operations at the expense of pull-down response time.
For example, in conventional chilled liquid systems using centrifugal compressors, when doing a “hot liquid” pull-down, that is, a pull-down where the liquid temperature is at or above the ambient temperature, the chilled liquid system is often operating at the current limit for the motor(s) of the compressor(s) of the chilled liquid system to pull or lower the leaving chilled liquid temperature (LCHLT) down to the operating setpoint. As the LCHLT asymptotically approaches the operating setpoint, the capacity controller for the chilled liquid system can unload the system by closing pre-rotation vanes to the compressor to minimize overshoot of the operating setpoint. When not operating at the current limit for the motor(s), the closing of the pre-rotation vanes would be the appropriate action since the chilled liquid system could work harder, for example, by increasing motor speed, if necessary, to pull the LCHLT down faster. However, when operating at the current limit for the motor(s), the chilled liquid system physically cannot work any harder to pull the LCHLT down. Since the chilled liquid system cannot work any harder, the LCHLT cannot be pulled down any faster and the rate of pull-down is slowed. When the capacity controller closes the pre-rotation vanes while the motor(s) are at or close to their corresponding current limits, the closing of the pre-rotation vanes can cause the pull-down time to be increased.